feat: Add --format to export command

[jkafrouni]

✨ Description of new changes

Fixes #24 by adding --format to the export command parser and setting the right underlying format if user selects .py or .ipynb, similar to how the import command supports format.
If no format is selected and the user is exporting a notebook, will continue using the default export format for notebooks which is ipynb.

[jkafrouni]

@microsoft-github-policy-service agree [company="Microsoft"]

[jkafrouni]

@microsoft-github-policy-service agree company="Microsoft"

[ayeshurun]

@jkafrouni please add test covering the export format scenario. You can use notebook .py formatting example in the test

[aviatco]

@jkafrouni in the description you meant --format and not --export am I correct?

[jkafrouni]

@jkafrouni in the description you meant --format and not --export am I correct?

Correct, my bad, just edited

[jkafrouni]

@jkafrouni please add test covering the export format scenario. You can use notebook .py formatting example in the test

Just added two tests for success and failure